Throughout this application various publications are referred to in parenthesis. Full citations for these references may be found at the end of the specification. The disclosures of these publications are hereby incorporated by reference in their entirety into the subject application to more fully describe the art to which the subject invention pertains.
From laboratory to clinic, directing therapeutics to specific sites (cells, tumors, tissues) and ensuring that the delivered agents will be active only at the desired sites remains a daunting challenge. Targeting is a particularly critical issue in cancer therapy because of the undesirable side effects of chemotherapeutic agents, many of which can also be toxic to normal cells.
Other technologies have attempted to engineer drug-delivery vehicles that act solely on targeted cells. For example, antibodies have emerged as important players in developing next generation cancer drugs, where they are being used not only as inhibitors, but also for targeted delivery of drugs, toxins and other molecules (reviewed in Carter and Senter 2008; Wu and Senter 2005). The strategy for using antibodies as targeting agents typically involves chemically conjugating the drug of choice to an antibody that recognizes a cell surface receptor. Most commonly, the targeted receptor is known to undergo receptor-mediated endocytosis, which leads to internalization of the targeted cargoes followed by release of the chemotherapeutic agent. While, this approach should be simple and effective, the use of antibodies as therapeutics is somewhat hampered by the need to engineer ‘humanized’ variants of these proteins that would not be immunogenic. Moreover, while numerous antibody drug conjugation strategies exist, almost all rely on conjugation through surface exposed amino groups (lysines), sulfhydryls (cysteines), or sugars (sialic acids) in these large (150 kDa) proteins. Precise control of drug placement, therefore, is nearly impossible and can compromise antibody function. The resultant conjugates are typically heterogeneous in nature, which can affect consistency in both production as well as efficacy.
The potential problems associated with using antibodies for targeted delivery of drugs has led to investigations into a variety of alternative targeting agents including antibody fragments such as Fabs, diabodies or single chain antibody fragments (Wu and Senter 2005). These engineered portions of antibodies do offer some advantages over full antibodies. In particular, their smaller size (25-40 kDa) aids in extravasation and tissue penetration. Additionally, they can be further engineered to facilitate some conjugation strategies; however, issues with immunity are still a factor. Small peptides, too, can be used for targeting cargoes. These small molecules, typically 8-10 amino acids in length, have generally been isolated using a method called phage display (McGuire et al. 2009); however, such reagents often require multivalent presentation for efficient targeting which can complicate their synthesis and production (Oyama et al. 2003).
The present invention provides an attractive alternative involving a novel system for the targeted delivery of drugs that specifically target tumor cells in whole animals. To do this, an alternate class of targeting molecules composed of nucleic acids called aptamer, internalizing nucleic acid or tumor-homing nucleic acids are used. The present invention also provides the molecules so-identified. These delivery aptamers, internalizing nucleic acids or tumor-homing nucleic acids are composed of cytotoxic nucleoside analogs and are engineered to target specific cells.